Optical networks transmit data over optical fiber. In an optical network, multiplexing protocols such as synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) may be used to transfer multiple digital bit streams over the same optical fiber or link. Lasers or light emitting diodes (LEDs) may be used to generate optical signals that carry the digital bit streams.
Bit streams traversing an optical network may pass through transponder switches. Such a switch may, for example, connect to multiple different fiber ports. Bit streams may be received at the switch, converted to an electrical signal, switched to the appropriate output port based on the electrical signal, converted back to an optical signal, and output as an optical signal on the determined output port.
Data transmitted over the optical links in the optical network may be encoded with error correction information. For example, an error-correcting code (ECC) may be added to the data. The ECC may include redundant data that is used to correct transmission errors in the data. If the number of errors experienced during transmission is within the capability of the ECC being used, the receiving transponder switch may use the extra information to discover the locations of the errors and correct the errors.
When a data signal is transmitted over an optical link, errors on the link may tend to occur in groups. For example, a multi-bit error that occurs over four consecutive bits may be more likely than four independent single bit errors. Although ECC information added to data may allow for the correction of single bit and some multi-bit error, at some point, too many errors in a codeword will prohibit correction of the error at the receiver.